Constant envelope modulation schemes employed in wireless communications, such as Gaussian minimum shift key (GMSK) modulation, involve direct current (DC) power requirements which are essentially independent of the data pattern(s) being transmitted. With non-constant envelope modulation schemes such as phase shift key (PSK) modulation with eight discrete amplitude levels (8-PSK), on the other hand, the maximum required power for power amplifiers varies depending on the data pattern. For certain data sequences, the maximum required direct current power is clearly above the normal average direct current power requirement. For instance, 8-PSK defines a crest factor (peak power divided by root mean square power) limit of 3.2 dB, which might lead to a peak power of 120 Watts (W) for a root mean square (RMS) power of 50 W, or peak power of 360 W for an RMS power of 150 W.
With such large variations in the required power, unless the voltage converter (AC/DC or DC/DC) for the power amplifier (other than Class A) has been designed for the absolute maximum peak power, system performance may be degraded when certain data patterns occur. As illustrated in FIG. 3, power requirements for random data patterns will normally average over a time slot 300, assuming a time division multiple access (TDMA) system is utilized. However, if the worst case data pattern (i.e., the data pattern requiring the highest peak output power) occurs during one time slot 301, the output voltage of the power amplifier may begin to drop during the next time slot 302. If successive time slots all contain the worst case data pattern, the voltage converter may not be able recover from the previous voltage drop and continue dropping the output voltage.
In addition to temporary service degradation, occurrence of the worst case data pattern, particularly in successive time slots, may result in the system shutting down. Additionally, repetition of the worst case data pattern may produce severe effects on the power amplifier performance and the voltage converter reliability, up to and including permanent damage to the equipment due to excessive current draw or long term thermal problems.
Since the probability of the worst case data pattern occurring at all—much less repeating successively—is usually very low, most power amplifier designs for wireless communications are based on average DC power at the maximum power level. However, multiple consecutive occurrences of the worst data pattern may still occasionally occur naturally. Moreover, the worst case data pattern may be calculated and synthesized for standardized systems, such as cellular networks. Such intentional generation of the worst case data pattern may result from a malicious purpose, by a competitor or individual desiring to disrupt the system and service.
There is, therefore, a need in the art to protect power amplifiers within wireless communications equipment from damage and to avoid system performance degradation or disruption.